Finger print sensors comprising electrodes for measuring characteristics in a finger surface are well known, e.g. EP0988614, U.S. Pat. Nos. 5,963,679 and 6,069,970 describe sensors based on different impedance or capacitance measurement principles with stripe shaped or matrix sensors comprising a number of individual sensor elements.
The sensor surfaces of current fingerprint sensors are in general not suitable for having extensive direct contact with the environment, and usually have to be provided with a housing protecting the circuits from humidity, wear, corrosion, chemical substances, electronic noise, mechanical influences, sun light electric discharges etc. U.S. Pat. No. 5,862,248 provides a possible solution to this problem, in which the circuit is enclosed in such a way that the finger is allowed to direct contact with the sensitive surface of the sensor through an opening in the top of the enclosure.
In many cases, this solution will not be sufficient to provide the required reliability. The materials (semiconductors, metal, dielectrics) being used on the surface of the integrated circuits are usually not sufficiently reliable to withstand exposure from the outer environment and contact with the finger over a longer period of time, and thus this solution will also lead to reliability problems. Another solution may be adding additional layers of metal and dielectrics on the chip surface, as described in U.S. Pat. No. 6,069,970. Adding thick dielectric layers, however, generally results in a degradation of the measurement signal and, hence, the performance of the fingerprint sensor. Such layers also tend to increase the production costs and create compatibility problems with the semiconductor process in general (related to processing temperature, varying dimensions due to temperature differences etc.). Yet another solution is described in EP1303828 routing the conductors through a substrate to the processor then being positioned safely on the other side of the substrate, inside the device.
Touch screen driven devices, such smart phones and tablet style personal computer devices, typically have a front surface where the display area is approaching 100% utilization and has very limited space available for physical buttons or other user interaction devices outside the display area. US 2013/0181949 A1 discloses one possible implementation of a transparent fingerprint sensor overlaying the touch screen of a smart phone. U.S. Pat. No. 8,564,314 disclose another possible implementation of a fingerprint sensor integrated with a capacitive touch position sensor where the sensor is positioned under glass. However, the patent does not discuss or propose any solutions to overcome the severe degradation of the sensor performance caused by the thick protective cover glass.